The invention relates to a method and plant for the cleaning and desulphurization of flue gas from the combustion of sulphurous fuels, and the recovery of heat from the flue gas, whereby the flue gas is conducted from a boiler through a heat exchanger.
When burning fossil fuels such as coal and oil in boiler plants, a flue gas arises which, in accordance with the relevant environmental regulations, must maintain stipulated norms in order to be able to be discharged into the atmosphere. The flue gas also contains a certain amount of thermal energy, even from boiler plants with economizers, whereby the feed water is heated by the flue gas before it is fed to the boiler plant. For reasons of economy, it is therefore desirable to recover as great an amount of the thermal energy as possible from the flue gas.
It is known to clean the smoke and recover a part of the thermal energy existing in the flue gas, in that one leads the flue gas from the economizer to a flue gas-air heat exchanger in which one heats the combustion air, which is to be fed to the boiler's combustion chamber.
Downstream of the heat exchanger, a smoke cleaner is provided in which a scrubber fluid, e.g. lime water, is used to remove acids and particles from the smoke, after which the flue gas can be conducted to the chimney and out into the atmosphere.
These known installations have several drawbacks. The smoke is cooled down considerably by using a flue-gas-air heat exchanger after the economizer, thus reducing the temperature of the flue gas to below the dew point of the acid, so that the acid is condensed and settles in the heat exchanger. This must therefore be made of an acid-resisting material, e.g. acid-resisting steel or glass, which results in a very expensive heat exchanger. As a rule, the flue gas from fossil fuels contains a smaller amount of lime connections, the result being that the particles in the flue gas become chalky and, in connection with the acid, thus form a gypsum paste which quickly blocks the heat exchanger. The heat exchanger must therefore be frequently cleaned, which is difficult. Known systems use vigorous flushing with water in order to clean the heat exchanger, but this gives rise to acidiferous water which must be neutralized before it can be led away. During the flushing periods, which are quite frequent, the heat exchanger cannot be used, the result being that the smoke must be conducted around the heat exchanger, whereby one loses some of the heat recovery, or a change must be made between two heat ecxhangers, which considerably increases the costs of the plant.
The aim underlying the present invention essentially rsesides in providing a method and a plant for cleaning and desulfurization of flue gas from the combustion of sulfurous fuels and recovery of heat from the flue gas which avoids, by simple means, shortcomings and disadvantages encountered in the prior art.
In methods and plants for cleaning a flue gas and recovery of heat from the same, the flue gas from a boiler is lead through a heat exchanger for water, a so-called economizer, and subsequently washed with a scrubber fluid before being lead to a chimney possibly by way of means which filter out drops of liquids with the thermal energy being collected in the scrubber fluid being reused.
According to the method of the present invention, the flue gas is cooled on its way through the economizer only so much that it still has a temperature above the acid dew point when the flue gas meets the scrubber fluid, and the scrubber fluid is maintained substantially at a neutral pH value by an addition of base in the liquid form, and an additive base is produced by the use of lime through a double alkaline process between the lime and the scrubber fluid in a high speed reactor wherein the pH value is maintained in a range of 8.5-10.5 and, preferably, at approximately 9.5.
In the plant or apparatus for cleaning of the flue gas of the present invention, a means is provided for feeding the gas flue through the economizer in such a manner that the flue gas still has a temperature above an acid dew point when the flue gas meets the scrubber fluid in the scrubbing elements, with a means for measuring the pH value of the scrubber fluid, with the measurement from the measuring means determining the addition of the liquid additive base from a tank so that the scrubber fluid is maintained substantially neutral, and means are provided for connecting the tank to a high speed reactor which is supplied with a precipitous sediment and liquid from the sediment tank with the sediment and liquid being led down through a substantially vertical downpipe together with a lime-water suspension from a mixing vessel.
The temperature of the flue gas is controlled in a known manner by using a motor valve to control the supply of water to the economizer. The flue gas from the boiler is led through the economizer and filtered direct to a smoke washer, in that one thus uses the smoke washer both for cleaning the smoke of acid and residual particles and also for a further cooling of the smoke, in that the scrubber fluid is heated by the flue gas, and the heated scrubber fluid is conducted, for example, to a water-air heat exchanger, a so-called calorifier, in which the combustion air for the combustion in the boiler is heated.
It is known to use various neutralization agents as base, e.g. lime, sodium hydroxide, ammonia etc., to hold the scrubber fluid neutral. Both ammonia and sodium hydroxide are bases which are easy to use and do not demand particularly high plant investments, but they have the disadvantage that the salt concentration becomes so great that it is necessary to divert much of the base and dilute it with water. The problem of air acidification is thus moved to become a discharge problem with contents of salts and heavy metals. From the operation point of view, both ammonia and especially sodium hydroxide are relatively expensive neutralization agents.
It is known to use lime in different ways as neutralization agent, either as a dry process, a dry/wet process or a wet process. The dry process has the disadvantage that the consumption of lime is quite large and increases progressively with increased reduction of the content of acid in the flue gas, to which must be added a greater amount of reject which has to be deposited. The dry/wet process reduces the consumption of lime and the reject in comparison to the dry process, but gives rise to cleaning problems between the dry and the wet zone because of the unavoidable formations of gypsum. The wet process's consumption of lime is linearly related to the amount of acid it is desired to remove, which means that the lime consumption and the reject is less than with the two other processes. The difficulties with the wet process are that the undissolved particles of lime result in the formation of gypsum crystals in the plant, which must therefore be cleaned very frequently.
With the present invention the known problems are avoided, while at one and the same time one achieves effective cleaning of the flue gas, a reduced consumption of lime and a reduction of the reject to an absolute minimum.
In accordance with further advantageous features of the plant of the present invention, a sieve is arranged upstream of the vertical pipe, a lowest portion of the high speed reactor includes a concave shield which is hit by the water/sediment jet and turns the stream upwards, and lime water is added to the water/sediment jet through the sieve by means of a dosing, which is controlled by a pH sensor in the high-speed reactor in such a manner that the pH-value in the high-speed reactor is held at approximately 9.0. By virtue of such arrangement, one is able to filter out crystals and larger solid particles, hereby avoiding packing of the reactor and blockage of the sludge outlet, and one can check the sieve visually and change it as required. One also achieves a very good mixing of the lime water with the water/sludge-liquid, the reason being that there is turbulent flow in the downpipe, so that the mixing takes place before the liquids reach into the actual reactor, in which there is a lower velocity of flow and therefore laminar flow. The turning-shield ensures that the speed of the water in the reactor immediately after the turning-shield is almost uniform throughout the whole of the cross-section of the high-speed reactor, except for the area along the wall. Finally, the clearance between the wall of the reactor and the turning-shield results in the sludge being able to pass down the inner wall of the high-speed reactor, past the turning-shield and down into the base cone of the reactor under the turning-shield. One thus achieves an easier separation of the lighter and/or smaller particles than is the case with known high-speed reactors, which is of great significance because the flue-gas cleaning process introduces many particles (sand, coke etc.) into the added water/sludge-liquid, and this leads to many small particles (crystals) being formed in the reactor.
It is also possible in accordance with the method of the present invention to treat the additive base, prior to being introduced into the scrubber fluid, by the addition of clean gas extracted after the flue gas is past through the scrubber fluid whereby the pH value of the other base is reduced to a substantially neutral value during simultaneous precipitation of possible remains of undissolved calcium hydroxide.
In the plant or apparatus of the present invention, a clean gas take off supplies clean gas extracted from the chimney through pipe lines to the additive base, with the base being stored in one or more collection vessels disposed between the high-speed reactor and the tank after which the clean gas is returned to the chimney.
By virtue of the last noted features of the present invention, all of the reject is sent back to the process and no diversion whatsoever of the solution to the discharge drainage system needs to be effected. Everything which is scrubbed from the flue gas and all of the lime which is added becomes a substantially dry shuffable residual product which can be easily deposited.
In order to provide advantageous regulation of the practice of the present invention and enable a full automation of the entire process, the pH value of the additive base is in the range of 6.9-7.6 when it is added to the scrubber fluid. The amount of clean gas which is added to the additive base is controlled by measuring the pH value of the additive base immediately before introduction into the scrubber fluid, and the amount of additive base which is introduced in the scrubber fluid is controlled by measuring the pH value of the scrubber fluid.
To realize the above mentioned regulation of the process of the present invention in the apparatus or plant, the tank in which the additive base is collected before being introduced into the scrubber fluid comprises a measuring element for the measurement of the pH value, with the measuring element being used to control a regulation element for the amount of clean gas which is added to the collection vessel.
Advantageously, the sedimentation tank contains a device for measuring the pH value, and the introduction of the additive base is controlled by the result of the measurement.
In accordance with still further features of the present invention, the sediment from the collection vessel or vessels and from the high-speed reactor is lead to a centrifuge, preferably a decanter centrifuge and the centrifuged liquid is returned to the system, for example, to the high-speed reactor and the substantially dry sludge from the centrifuge is collected in a container.
In accordance with still further features of the plant or apparatus of the present invention, the sedimentation tank, the scrubber element and the remaining parts which come into contact with the scrubber fluid, for example, the flushing jets, scrubber elements, etc. are made of or cased with glass, porcelain or other acid or base proof materials whereby the overall lifetime and operational reliability of the plant or apparatus is increased by virtue of the elimination of the formation of gypsum crystals on these materials.